Aluminum base alloy process and product



3,539,405 ALUMINUM BASE ALLOY PROCESS AND PRODUCT George S. Foerster, Midland, Mich., assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. Filed Apr. 1, 1968, Ser. No. 717,910 Int. Cl. 1522f 9/00; C221? 1/04 U.S. Cl. 148--12.7 9 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to a high strength, stress corrosion resistant aluminum alloy product and a method of preparing the alloy product which comprises providing an aluminum base alloy of defined composition containing magnesium and silicon and optionally one or more dispersion hardening elements; atomizing the alloy composition to form pellets; working the so-pelleted alloy; heat-treating the Worked alloy within a temperature range of from about 1000 F. to about 1080 F.; and quenching and aging the heat treated alloy, thereby producing a high strength, stress corrosion resistant alloy product.

BACKGROUND OF THE INVENTION Field of the invention The present invention concerns a high strength, stress corrosion resistant alloy product and a method of preparing said alloy product.

Prior art Conventional high strength aluminum base alloys are prone to stress corrosion even at stresses well below their tensile yield strength. For example, the ASTM designated 7075 aluminum base alloy containing about 5.5 percent zinc, 2.5 percent magnesium, 1.5 percent copper, and 0.3 percent chromium, while having a tensile yield strength of about 80 thousand pounds per square inch (K s.i.), fails rapidly, e.g. within a few days, in stress corrosion at stresses of about 50 K s.i. or less. On the other hand, aluminum alloys containing magnesium and silicon have excellent stress corrosion resistance but only moderate strength. For instance, an aluminum base alloy containing about 1.2 percent magnesium, and 0.8 percent silicon has a tensile yield strength on the order of about K s.i. but can withstand stresses near this level for a significant period of time, e.g., several months without failure due to stress corrosion.

An object of the present invention is to provide an aluminum base alloy product which has excellent stress corrosion resistance as well as relatively high strength.

Another object of the present invention is to provide a process for preparing a relatively high strength, stress corrosion resistant aluminum base alloy product.

THE INVENTION In accordance with the present invention the above and other objects and advantages are found in a novel aluminum base alloy product and a method of preparation which comprises: providing an aluminum base alloy consisting essentially by weight of from 1 to about 10 weight percent magnesium silicide (Mg si), preferably from about 2 to about 5 percent Mg Si, and optionally one or more dispersion hardeners, the balance being aluminum; atomizing said alloy, such as by jet atomizing, to form pellets, so as to produce a fine second phase dispersion in the aluminum matrix; hot working the pelleted alloy, such as by extruding at elevated temperatures; heat treating the Worked alloy at a temperature of United States Patent 0 ice from about 1000 F. to 1080 F. for a time sufficient to dissolve essentially all the soluble Mg Si; quenching the alloy; aging the alloy for from about 1 to about 50 hours at a temperature within the range of 275 F. to about 450 F., thereby producing a stress corrosion resistant, relatively high strength aluminum alloy product.

In preparing the alloys to be provided in the method of the present invention, conventional alloy and melting techniques, as practiced by those skilled in the art, may be employed. The alloying constituents may contain those amounts and types of impurities normally found therein. Preferably, in addition to magnesium and silicon in the proportions to form Mg Si, at least one dispersion hardener, as that term will hereinafter be defined, is added as an alloying element for even further increases in strength properties of the alloy product. When one or more dispersion hardeners are employed, additional silicon may be required to form the Mg Si because silicon may preferentially combine with the dispersion hardener.

The term dispersion hardener as used herein refers to alloying elements which are slow diffusing and slightly soluble in solid aluminum, such as, chromium, iron, manganese, nickel and rare earth elements, and which are present in an aluminum alloy in an amount suflicient to produce a dispersed phase in the aluminum matrix of from about 0.5 to about 20 volume percent, preferably from about 5 to about 15 volume percent. Lower amounts of dispersion hardeners produce little additional strength ening. Higher amounts of dispersion hardeners tend to decrease ductility excessively. It is well within those skilled in the alloying art to select the amount and type of dispersion hardener for the desired combination of properties.

A melt of the above defined alloy is conventionally atomized into pellets, having a particle size of less than about 20 mesh, and preferably less than about mesh, as by jet atomizing or wheel atomizing either in an inert atmosphere, such as natural gas or argon, or in air.

The pellets can be hot worked by extrusion, forging or other conventional pellet fabricating technique at a temperature within the range of from about 600 F. to about 1000" F. to form an alloy product characterized by a fine second phase dispersion in the aluminum matrix, i.e., a dispersion hardened aluminum base alloy. Preferably the pellets are worked by extrusion.

Solution heat treatment of the Worked product is accomplished by heat treating at ultra high temperatures, i.e., about 1000" F. to about 1080 F., preferably from about 1040 F. to about 1075 F., well above conventional heat treating temperatures, e.g., about 940 F. to 970 F., for a sufiicient time to put essentially all of the soluble Mg Si second phase into solution. This can usually be accomplished very rapidly, e.g., a few minutes or at least less than 60 minutes. In fact, just heating the alloy to the desired temperature may be adequate. Excess time at temperature is not desired because of agglomeration of the dispersion.

The heat treated alloy product can be conventionally quenched in Water, oil or other quenching medium to room temperature and aged at a temperature within the range of from about 275 F. to about 450 F. for from about 1 to about 50 hours.

The aluminum alloy product produced by the novel method of the present invention possesses the heretofore unknown, highly desirable combination of excellent stress corrosion resistance and relatively high tensile strength. This is apparently due to the high degree of energy retained in the particular family of aluminum alloys herein defined from rapid solidification and hot working and to the ultra high temperature heat treatment producing more effective precipitation hardening of Mg Si in an aluminum matrix.

3 The following examples are representative of the present invention and are not to be construed as limiting the invention thereto.

EXAMPLES l8 A number of aluminum 'base alloy melts having compositions Within the present invention were prepared and conventionally atomized into pellet form. Separate batches of pellets were preheated to about 6-50900 F., placed in the container of a ram extruder and extruded into 0.2" x 1" strip at 600'850 F. at a speed of feet per minute.

The resulting extruded samples were solution heat treated at 970 F., 1000 F., 1040 F. and 1075 F. for /2 hour (20 minutes at temperature). The samples wgzre quenched and aged for about 8 hours at about 3 0 F.

For comparison, melts of alloys having compositions within the present invention were formed into ingots, and extruded, heat treated, quenched and aged similar to the pellets.

Standard test bars were prepared and the percent elongation (percent E), tensile yield strength (TYS) and tensile strength (TS) of the pellet and ingot extrusions were determined at room temperature. The results are the method of the present invention having relatively high strength withstood the above stress corrosion environment at stresses close to the alloys tensile yield strength for over six months without failure at which time the test was concluded.

For comparison an aluminum base alloy containing about 6 percent zinc, about 2 percent magnesium, about 2 percent copper, and 2 percent manganese was atomized, the resulting pellets extruded in A; inch diameter rod at 800 F. at 5 f.p.m. and the rods conventionally heat treated at 870 F. for /2 hour, quenched in H 0, and aged 2 days at 250 F. The resulting properties measured were 6%, 93 K s.i. TYS, and 100 K s.i. TS. C-rings were prepared and tested for stress corrosion under similar conditions to Example 7 above. C-rings of this dispersion hardened aluminum base alloy, not within the alloy composition of the present invention and not prepared by the present invention method, although having excellent strength properties, failed within about 1 to 3 days at 50 K s.i. stress and within about 1 week at 40 K s.i. stress, i.e. a stress less than half the alloys tensile yield strength.

A second comparison aluminum base alloy containing about 5.5 percent zinc, about 2.5 percent magnesium, about 1.5 percent copper, and 0.3 percent chromium was shown in the table. conventionally melted and cast into a 4 inch diameter TABLE Heat treatment Composition 1 970 F. 1,000 F. 1,040 F. 1,075 F.

Percent Percent Percent Percent Percent Percent Percent Example No. Mg S other E TYS E TYS E TYS E TYS TS Comparative (ingot) 1. 2 6 50 53 6 45 48 8 45 49 9 46 50 Do- 1. 2 15 37 42 19 31 40 20 33 42 19 36 45 1. 2 10 16 28 38 16 32 40 14 36 44 1. 2 14 44 46 I4 40 11 44 47 10 47 51 1.2 6 46 52 4 48 52 6 51 56 2 53 56 1. 2 4 56 62 4 59 65 5 64 68 4 66 68 1. 2 7 52 5s 6 55 59 6 58 61 2 53 56 1.2 7 51 57 8 54 59 8 59 63 2 61 66 1.2 7 8 54 8 57 62 4 58 66 1. 5 4 52 61 2 62 1 59 62 a) (a) 7. 5 2 52 56 1 55 59 0. 5 5s 59 a a (a) 1 The balance being essentially aluminum. 2 Misch metal, mixture of rare earth elements. 3 N 0t measured.

The data of the table demonstrate the necessity of the combination of alloy composition, dispersion hardening and heat treatment as taught by the present specification. Heat treating at ultra high temperatures is ineifective, and in most cases detrimental, to strength properties of the ingot extrusions, i.e., not atomized according to the present invention even though the alloys were within the composition limits of the present alloy product. However, when alloys within the composition limits heretofore defined are dispersion hardened by the process steps within the present invention method as by atomizing and hot Working, heat treatment at ultrahigh temperatures, i.e., about 1000 F. to about 1080 F., is extremely efiective in producing a relatively high strength aluminum alloy product.

EXAMPLE 9 An aluminum base alloy consisting essentially of about 1.2 percent magnesium, about 2.5 percent silicon and 3.0 percent manganese, the balance being essentially aluminum, was atomized and the pellets extruded into 78 inch diameter rod at 800 F., at 5 f.p.rn. The rod was heat treated for about /2 hour at 1050" F., quenched in Water and aged about 16 hours at about 320 F. The resulting properties measured were 8% E, 61 K s.i. TYS and 67 K s.i. TS. C-rings having 0.600 inch outer diameter and 0.484 inch inner diameter were machined with their axes parallel and perpendicular to the extrusion axis. They were stressed at 50 K s.i. in the transverse direction and subiected to alternate immersion, about 10 minutes in solution and 5 minutes out of solution, in a 3 /2 percent NaCl solution. This aluminum alloy product prepared by ingot. The ingot was extruded from a 4 inch diameter container at l f.p.m. at 830 F. into a inch square. The resulting extrusion was conventionally heat treated at 870 F. and aged at 250 F. The resulting properties were 11% E, 85.6 K s.i. TYS, and 90.5 K s.i. TS. C-rings were prepared and tested for stress corrosion under similar conditions to Example 7 above. C-rings of this dispersion hardened aluminum base alloy, not within the alloy composition of the present invention and not prepared by the present invention method, although having excellent strength properties, failed within about 1 to 2 days at 50 K s.i. stress.

Again these comparative and working examples demonstrate the unique, highly critical combination of alloy composition and process steps required to produce the high strength, stress corrosion resistant aluminum base alloy product of the present invention.

The present invention may be modified or changed without departing from the spirit or scope thereof and it is understood that the invention is only limited as defined in the appended claims.

What is claimed is:

1. A process for preparing a high strength, stress corrosion resistant alloy product which comprises:

(a) atomizing an alloy consisting essentially of aluminum containing from about 1 to about 1'0 weight percent magnesium silicide into pellets having a fine second phase dispersion of said magnesium silicide in the aluminum matrix;

(b) hot working, at a temperature within the range of from about 600 F. to about 1000 F., the p llets into an aluminum alloy product;

(c) heat treating the worked alloy product at a temperature within the range of from about 1000 F. to about 1080 F. for a time sufiicient to put essentially all of the soluble magnesium silicide in said product into solution;

(d) quenching the heat treated alloy product;

(e) aging the alloy product at a temperature within the range of from about 275 F. to about 450 F. for from about 1 to about 50 hours, thereby to produce a high strength, stress corrosion resistant aluminum alloy product.

2. The process of claim 1 wherein the aluminum base alloy contains, in addition to magnesium silicide at least one dispersion hardener in an amount sufficient to produce from about 0.5 to about 20 volume percent dispersed second phase in the aluminum matrix.

3. The process of claim 2 wherein the dispersion hardener is selected from the group consisting of chromium, iron, manganese, nickel and rare earth elements.

4. The process of claim 1 wherein the amount of magnesium silicide in the aluminum base alloy is from about 2 to about 5 weight percent.

5. The process of claim 4 wherein the aluminum base alloy contains, in addition to magnesium silicide, at least one dispersion hardener in an amount suificient to produce 6 from about 5 to about 15 volume percent dispersed second phase in the aluminum matrix.

6. The process of claim 1 wherein in step (a) the atomizing is accomplished by jet atomizing using an inert atmosphere.

7. The process of claim 1 wherein in step (b) the hot Working is accomplished by extrusion.

8. The process of claim 1 wherein in step (c) the heat treating is carried out at a temperature within the range of from about 1040 F. to about 1075 F.

9. A high strength, stress corrosion resistant aluminum base alloy product prepared in accordance to the process of claim 1.

References Cited UNITED STATES PATENTS 4/1965 Foerster 14832.5 12/1965 Baugh et a1 148-32.5

US. Cl. X.R. 14832.5 

